A microelectronic unit, such as a semiconductor chip, is typically connected to an external circuit element through contacts accessible at a surface of the microelectronic unit. For example, in a so-called "flip chip" process, solder balls are provided directly on the contacts of a bare semiconductor chip, so that the solder balls project from the front or contact-bearing surface of the chip. The bare chip is placed front-side down on a circuit board, so that the solder balls rest on contact pads of circuit board, also referred to as a printed wiring board or "PWB". The solder is "reflowed" by bringing the assembly to a temperature above the melting temperature of the solder and then allowing the assembly to cool. The solder forms connections between the contacts on the chip and the contact pads on the PWB. In a variant of this process, the solder balls are provided on the PWB, rather than on the chip.
As described, for example, in certain embodiments of commonly assigned U.S. Pat. Nos. 5,148,265; 5,148,266; and 5,518,964, the disclosures of which are hereby incorporated by reference herein, packaged semiconductor chips can be provided with a packaging element having terminals thereon, the terminals being connected to the contacts of the chip. In certain embodiments described in these patents the terminals are solder-bonded to other elements, such as to contact pads on a circuit board, by processes generally similar to those used in flip-chip bonding of a bare chip. Thus, solder balls can be placed on the terminals of the packaged semiconductor chip and the resulting assembly can be placed onto a PWB. As described in preferred embodiments of these patents, the terminals of the package may be movable with respect to the chip. This minimizes stress on the solder balls during service. As further described in preferred embodiments of U.S. Pat. No. 5,801,446, the disclosure of which is hereby incorporated by reference herein, generally similar structures can be made using solid core solder balls, i.e., solder balls having a center or core formed from copper or another material which does not melt during the solder reflow operation.
In still other processes, commonly referred to as tape automated bonding or "TAB", a flexible dielectric sheet, such as a thin foil of polyimide, includes conductive terminals accessible at a surface thereof and flexible metallic leads connected to the terminals. The flexible dielectric sheet also preferably includes one or more bond windows extending therethrough. Each flexible lead has a first end integrally connected to one of the conductive terminal and a second end remote therefrom which projects over one of the bond windows. The flexible dielectric sheet is typically juxtaposed with a semiconductor chip so that the bond windows are aligned with contacts on a front end face of the chip and so that the second ends of the leads overlie the contacts. The flexible leads may then be bonded to the chip contacts using bonding techniques, such as ultrasonic or thermocompression bonding. After the bonding step, the resulting chip package may be electrically interconnected with an external circuit element, such as a PWB, by a similar solder-bonding process in which the solder balls are reflowed and solidified. The resolidified solder balls both mechanically and electrically interconnect the chip contacts with the contact pads on the external circuit element.
Similar procedures are used to mount connect other components with one another. The terms "land grid array" and "ball grid array" are sometimes used to describe mounting structures which use arrays of solder balls on one of the components to be connected. All of these procedures require that solder balls be placed onto contacts, terminals or other conductive features. For simplicity, the term "contact" is used in this disclosure as referring to any conductive feature which is to receive a mass of a bonding material such as a solder ball. Also, the term "microelectronic unit" is used herein as referring to any device having contacts thereon, including without limitation bare or packaged semiconductor chips, semiconductor wafers, and PWB's.
One common method of placing solder balls onto contacts of a microelectronic unit is to use a solder ball stencil having apertures arranged in a pattern corresponding to the pattern of contacts on a microelectronic unit. The apertures are just slightly larger in diameter than the solder balls. First, a pasty flux composition is applied to the contacts. Then, the solder ball stencil is disposed over the contact-bearing surface of the microelectronic unit, and the apertures of the stencil are aligned with the contacts on the microelectronic unit. Solder balls are disposed on top of the stencil and swept over the top of the stencil using a squeegee or brush, so that one solder ball falls into each aperture and onto each contact. Any excess solder balls are swept off of the stencil, whereupon the stencil is removed, leaving only one solder ball behind on each contact and held by the flux. The solder balls are then reflowed. Processes of this type encounter difficulties when placing small solder balls, such as solder balls less than about 1500 .mu.m in diameter, and particularly when placing solder balls less than about 500 .mu.m in diameter. Such small solder balls are required for use with compact microelectronic units having closely-spaced contacts.
As described in certain preferred embodiments of commonly assigned, copending U.S. patent application Ser. No. 09/086,808, the disclosure of which is hereby incorporated by reference herein, the solder ball stencil may have a main body portion defining the apertures and spacer elements disposed on the bottom of the main body portion. The spacer elements hold the main body portion up above the surface of the microelectronic unit, and thereby minimize contamination of the main body portion by flux. As also described in the '-808 application, excess solder balls may be retained in a reservoir used in conjunction with the solder ball stencil. Also, the top surface of the stencil may be disposed at a height above the contacts which is about equal to the diameters of the solder balls. With this arrangement, only one solder ball should be received in each aperture of the stencil, and hence only one solder ball should be deposited on each contact.
Despite the improvement in the art represented by the '-808 application, still further improvement would be desirable.